Plated steel sheet having excellent fusion resistance, and manufacturing method therefor

ABSTRACT

A plated steel sheet having excellent fusion resistance, and a manufacturing method. therefor are provided. Provided is a hot forming plated steel sheet having a plated layer formed on one surface or the both surfaces of a base steel sheet and having excellent fusion resistance, wherein a surface layer portion of the plated layer is comprised of an hard alloy layer with a surface area ratio of 1% or more including hard alloy phases, the hard alloy layer having a thickness of 0.1-100 μm, and a balance of a soft plating layer, wherein the hard alloy phases comprise Al, Zn, Mg, Si, Fe and a balance of unavoidable impurities, a sum of Al, Zn and Fe being 70 wt % or more on the basis of weight % thereof.

TECHNICAL FIELD

The present disclosure relates to a technology for manufacturing a plated steel sheet, and more particularly, to a plated steel sheet having excellent fusion resistance and a method of manufacturing the same.

BACKGROUND ART

Plated steel sheets manufactured by hot dip plating and electroplating are mainly used as steel sheets for interior and exterior construction, home appliances, and automobiles based on excellent corrosion resistance.

In detail, in the automotive field, when hot forming is performed using a plated steel sheet made of high alloy steel as a material, a remarkable improvement in strength may be obtained, and the amount of use thereof is increasing as a vehicle body or structural member.

However, in the hot forming process of 700° C. or higher, plating components with a relatively low melting point are often melted and adhered to the mold, which causes damage to the mold, damage to the molded product, and a decrease in corrosion resistance of the molded product due to the loss of a plating layer, and a decrease in productivity due to periodic cleaning of the mold.

The inventions described in Patent Document 1 and Patent Document 2 may be mentioned as the technique for solving the problems of the prior art. Patent Documents 1 and 2 disclose a method of reducing friction with a mold by applying an organic resin on the surface of a plated steel sheet, but there are disadvantages of lowering weldability, contamination of the mold due to the detachment of an organic resin, and an increase in manufacturing costs.

PRIOR TECHNICAL LITERATURE Patent Literature

-   (Patent Document 1) Korean Patent Application No. 10-2015-0080014 -   (Patent Document 2) Korean Patent Application No. 10-2015-0080012

DISCLOSURE Technical Problem

Therefore, an aspect of the present disclosure is to provide a plated steel sheet having excellent fusion resistance without applying a separate organic resin to the plated steel sheet.

Another aspect of the present disclosure is to provide a method of manufacturing a plated steel sheet having excellent fusion resistance without applying a separate organic resin thereto.

In addition, the technical problems to be solved in the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those of ordinary skill in the art from the following description.

Technical Solution

According to an aspect of the present disclosure, a plated steel sheet for hot forming having excellent fusion resistance, includes a plated layer disposed on one surface or both surfaces of a base steel sheet,

wherein a surface layer portion of the plated layer is comprised of an hard alloy layer with a surface area ratio of 1% or more including hard alloy phases, the hard alloy layer having a thickness of 0.1-100 μm, and a balance of a soft plating layer,

wherein the hard alloy phases comprise Al, Zn, Mg, Si, Fe and a balance of unavoidable impurities, a sum of Al, Zn and Fe being 70 wt % or more on the basis of weight % thereof.

The hard alloy layer may have an area ratio in a range of 15 to 100%.

The soft plating layer may include Al, Zn, Mg, Si and a balance of unavoidable impurities, a sum of Al and Zn being 60% or more on the basis of weight % thereof.

According to an aspect of the present disclosure, a method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance, includes:

immersing a base steel sheet in a plating bath and plating a surface thereof; and

tempering rolling the plated steel sheet discharged from the plating bath and having a plating layer formed on the surface thereof,

wherein, in the tempering rolling, hot tempering rolling is performed at a pressure of 10 to 50,000 MPa while heating the plated steel sheet at a temperature ranging from 300 to 700° C.

The plating layer may include Al, Zn, Mg, Si and a balance of unavoidable impurities, a sum of Al and Zn being 60% or more on the basis of weight % thereof.

A surface layer portion of a plated layer of the temper-rolled plated steel sheet may be comprised of an hard alloy layer with an area ratio of 1% or more having a thickness of 0.1 to 100 μm and including hard alloy phases, and a soft plating layer as a remainder thereof,

wherein the hard alloy phases include Al, Zn, Mg, Si, Fe, and a balance of unavoidable impurities, a sum of Al, Zn and Fe being 70% or more on the basis of weight % thereof. The hard alloy layer may have an area ratio in a range of 15 to 100%.

The plating may be performed by immersing the base steel sheet in a hot-dip plating bath and forming a hot-dip plated layer on a surface of the base steel sheet.

Advantageous Effects

According to an exemplary embodiment of the present disclosure with the above-described configuration, on a surface layer portion of a plating layer of a plated steel sheet, a hard alloy layer including Al, Zn, Mg, Si and Fe, in which the sum of Al, Zn and Fe is 70% or more based on weight % thereof, and which has a thickness of 0.1 to 100 μm, may be formed. Further, since the hard alloy layer may have an area ratio of 1% or more, a plated steel sheet for hot forming, having excellent fusion resistance, may be provided.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a cross section of a plated layer of a plated steel sheet having undergone tempering rolling of the related art.

FIG. 2 is a diagram illustrating a cross section of a plated layer of a plated steel sheet subjected to tempering rolling according to an exemplary embodiment of the present disclosure. FIG. 2A is a drawing illustrating a case in which a portion of a plating layer remains in a portion of an alloy layer formed on a surface layer portion of the plating layer and FIG. 2B is a diagram illustrating a case in which the alloy layer is all formed without remaining plating layer, by tempering rolling.

FIG. 3 is an actual cross-sectional image of a plated layer of a plated steel sheet, in which FIG. 3A is a cross-sectional image of a plating layer of a plated steel sheet having undergone tempering rolling in the related art, and FIG. 3B is a cross-sectional image of the plating layer of the plated steel sheet subjected to tempering rolling according to an exemplary embodiment of the present disclosure.

BEST MODE FOR INVENTION

Hereinafter, an exemplary embodiment of the present disclosure will be described.

In the case of a related art plated steel sheet for hot forming, there is a problem in which the plating layer partially dissolves when heated at a high temperature for hot forming and adheres to the mold during hot forming as described above. Therefore, the present inventors have repeatedly researched solving the problems of the prior art, and as a result, have confirmed that a plated steel sheet having excellent fusion resistance may be manufactured during hot forming, by controlling the structure of a surface layer portion of the plating layer into a hard alloy layer that is not easily dissolved at high temperatures before heating the plated steel sheet to high temperature for hot forming, thereby presenting the present disclosure.

Therefore, in the case of a plated steel sheet for hot forming having excellent fusion resistance according to an exemplary embodiment of the present disclosure, in a plated steel sheet in which a plating layer is formed on one or both surfaces of a base steel sheet, a surface layer portion of the plating layer is formed of an alloy layer that has a thickness of 0.1 to 100 μm and has a surface area ratio of 1% or more and includes hard alloy phases, and a remaining soft plating layer.

Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings.

FIG. 1 is a diagram illustrating a cross section of a plated layer of a plated steel sheet that has undergone tempering rolling of the related art. FIGS. 2A and 2B are diagrams illustrating cross sections of a plated layer of a plated steel sheet subjected to tempering rolling according to an exemplary embodiment of the present disclosure. FIG. 2A is a diagram illustrating a case in which a portion of a plating layer remains in a portion of an alloy layer formed on a surface layer portion of the plating layer by tempering rolling, and FIG. 2B is a diagram illustrating a case in which no plating layer remains and all of the plating layer is entirely formed of alloy layer.

As illustrated in FIG. 1, the related art plated steel sheet has a structure in which a hard alloy layer 13 is formed between a base steel sheet 11 and a surface plated layer 15. It is well-known in the art that the hard alloy layer 13 as described above is an intermetallic compound formed by a reaction between a component metal constituting the plating layer and a base iron component, or the like. In contrast, in the case of a plated steel sheet using a tempering rolling process according to an exemplary embodiment, a hard alloy layer 23 is formed on a base steel plate 21 as illustrated in FIGS. 2A and 2B.

FIGS. 3A and 3B are actual cross-sectional images of the plated layer of the plated steel sheet. FIG. 3A is a cross-sectional image of a plating layer of a plated steel sheet having undergone tempering rolling of the related art, and FIG. 3B is a cross-sectional image of the plating layer of the plated steel sheet subjected to tempering rolling according to an exemplary embodiment of the present disclosure.

In detail, a plated steel sheet for hot forming according to an exemplary embodiment of the present disclosure includes the base steel sheet 21.

In the present disclosure, it may be preferable to use carbon steel or the like as the base steel sheet, but the present disclosure is not limited thereto, and materials having various compositional compositions may be used.

In addition, the plated steel sheet for hot forming according to an exemplary embodiment of the present disclosure includes the hard alloy layer 23 formed on a surface layer portion of the plated layer of the base steel sheet 21, as illustrated in FIG. 2B.

In the present disclosure, the alloy layer 13 formed between the base steel sheet 11 and the plating layer 15 in the related art is rapidly grown to the surface of the plating layer through hot tempering rolling in advance as illustrated in FIG. 1, and as illustrated in FIG. 2B, it is characterized in that a plated steel sheet in which the hard alloy layer 23 is formed on the surface layer portion of the plated layer of the base steel plate 21 is provided. Through this hot tempering rolling process, the plating layer is changed into a hard alloy layer, and thereafter, and then, in the hot forming process of 700° C. or higher, the alloying reaction between the plating layer and the mold is prevented, such that fusion resistance may be improved.

In an exemplary embodiment of the present disclosure, the surface layer portion of the plating layer may be comprised of the alloy layer 23 with an area ratio of 1% or more having a thickness of 0.1 to 100 μm and including hard alloy phases, and the remaining soft plating layer 25, as illustrated in FIG. 2A.

In the present disclosure, if the area ratio of the hard alloy layer is less than 1%, the reaction with the mold may increase at relatively high temperature. If the thickness of the alloy layer 23 is less than 0.1 μm, the diffusion of the plating layer may not be suppressed, such that fusion resistance is insufficient. If the thickness exceeds 100 μm, even when hot tempering rolling is performed, the hard alloy layer 23 may not be able to grow sufficiently to the surface of the plating layer.

In an exemplary embodiment of the present disclosure, the hard alloy layer is more preferably controlled to have an area ratio in the range of 15 to 100%, even more preferably 50 to 100%, most preferably, the hard alloy layer having an area ratio of 100% (in the case in which all the plating layers are changed into alloy layers).

In addition, in an exemplary embodiment, the hard alloy phases contain Al, Zn, Mg, Si, Fe, and residual unavoidable impurities, and it may be preferable that the sum of Al, Zn and Fe is 70% or more, on the basis of weight % thereof. If the sum of Al, Zn, and Fe is less than 70% by weight, sufficient corrosion resistance may not be secured, and in this case, metal vaporization of the plating layer may occur during the high-temperature molding process.

In addition, in the present disclosure, as described above, the surface layer portion of the plating layer may include the alloy layer 23, and a soft plating layer 25 that has not changed to an alloy layer despite tempering rolling as a residual component, as illustrated in FIG. 2A. In the present disclosure, the plating layer 25 contains Al, Zn, Mg, Si, and unavoidable impurities, and the sum of Al and Zn may be preferably 60% or more on the basis of weight % thereof.

On the other hand, the present disclosure may also be applied to a plated steel sheet in which a plating layer is formed on both surface of the base steel plate, as well as to a plated steel sheet in which the plating layer is formed on one surface.

Next, a method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance according to an exemplary embodiment of the present disclosure will be described.

The method of manufacturing a plated steel sheet for hot forming according to an exemplary embodiment includes: putting a base steel plate into a plating bath and plating the surface thereof; and tempering rolling a plated steel sheet discharged from the plating bath and having a plating layer formed on the surface thereof. In the tempering rolling process, tempering rolling of the plated steel sheet is performed at a pressure of 10 to 50,000 MPa while heating the plated steel sheet at a temperature ranging from 300 to 700° C.

The process of manufacturing a plated steel sheet using a hot dip plating method according to an embodiment of the present disclosure may roughly include processes such as pretreatment, heat treatment, plating, hot tempering rolling, shape correction, post-treatment and the like of the base steel plate.

In the present disclosure, before performing plating by immersing the base steel plate in a plating bath, a pretreatment operation of washing the surface of the base steel plate may be performed if necessary, using a chemical, physical or electrochemical method.

In addition, a heat treatment process of heating the pre-treated base steel plate may be performed as necessary, which gives the steel sheet, mechanical properties suitable for the usage, and also enables uniform plating by maintaining the same temperature as the molten plating material in the subsequent hot dip plating process.

Subsequently, in the present disclosure, a plated layer is formed on the surface of the base steel plate by immersing the base steel plate subjected to the pretreatment and the like in the plating bath to coat the surface of the base steel plate.

In the present disclosure, a hot-dip plating method or an electroplating method maybe used as the plating method, and preferably, a hot-dip plating method may be used.

The plating layer formed through the plating may include Al, Zn, Mg, Si and residual unavoidable impurities, and the sum of Al and Zn may be preferably 60% by weight or more on the basis of the weight % thereof. This is because Al and Zn are the main components of the hard alloy layer, and if the sum of Al and Zn is less than 60%, the hard alloy layer may not be sufficiently formed even in hot tempering rolling, and corrosion resistance by the passivation film of Al and the sacrificial method of Zn may not be sufficiently secured.

Thereafter, in the exemplary embodiment, the plated steel sheet on which the plating layer is formed is hot temper rolled.

In the related art, simple tempering rolling is performed at room temperature in order to adjust the surface finish of the plating layer of the plated steel sheet discharged from the plating bath and to remove the elongation at the yield point. However, to this end, to rapidly grow the hard alloy layer to the surface layer of the plating layer in an exemplary embodiment of the present disclosure, a heating device is added to the tempering rolling device to perform high temperature and high pressure.

In detail, in an exemplary embodiment, it may be preferable to control the range of the hot tempering rolling temperature to be in 300 to 700° C. If the hot tempering rolling temperature is less than 300° C., the hard alloy layer may not be sufficiently grown, and if it exceeds 700° C., the plating layer may be fused with the tempering rolling roll.

On the other hand, in the present disclosure, both direct and indirect heating methods may be used as a heating method using the heating device, and the heating method is not limited to a specific heating method.

In addition, in an exemplary embodiment, it may be preferable to control the hot tempering rolling pressure to be 10 to 50,000 MPa. If the hot tempering rolling pressure is less than 10 MPa, surface finish and yield point elongation may not be sufficiently controlled, and if it exceeds 50,000 MPa, work hardening occurs and it may be difficult to secure mechanical properties suitable for the required specifications.

In addition, the surface layer portion of the plating layer of the hot temper-rolled plated steel sheet may be formed of an alloy layer with a surface area ratio of 1% or more having a thickness of 0.1 to 100 μm and including hard alloy phases, and a remaining soft plating layer.

In an exemplary embodiment, more preferably, the hard alloy layer maybe controlled to have an area ratio in the range of 15 to 100%, even more preferably 50 to 100%. Most preferably, the hard alloy layer may have an area ratio of 100%.

In addition, the hard alloy phase includes Al, Zn, Mg, Si, Fe, and residual unavoidable impurities, and the sum of Al, Zn, and Fe may be 70% or more in weight percent.

Subsequently, in the present disclosure, if necessary, a shape correcting process may be performed using a shape corrector to correct defects such as curvature of the plated steel sheet or the like.

Furthermore, after the shape correction process, if necessary, a temporary rust inhibitor may be applied onto the plated steel sheet according to the required specification, and then, the produced plated steel sheet product may be packaged for easy transportation and handling.

Mode for Invention

Hereinafter, the present disclosure will be described in detail through examples.

EXAMPLE

Each steel plate in the form of a plate having a thickness of 1.0 mm was prepared using a hot dip plating simulation device.

The steel plates prepared as described above were immersed in the hot-dip plating bath, in which Al, Zn, Mg and Si are put and the weight ratio (%) of Al+Zn is controlled as illustrated in Table 1 below, and then, the prepared steel plates were removed therefrom, to prepare a plated steel sheet having a hot-dip plated layer formed on the surface thereof.

Thereafter, each of the plated steel sheets prepared as described above was hot temper-rolled under the conditions illustrated in Table 1 below to obtain a plated steel sheet having a hard alloy layer formed on a surface layer portion of a plated layer.

TABLE 1 Hot tempering rolling conditions Classification Al + Zn Wt (%) Temperature (° C.) Pressure (MPa) Example of 90  25 5,000 Related art Comparative 90 400 5 Example 1 Comparative 50 400 5,000 Example 2 Comparative 50 750 60,000 Example 3 Inventive 60 400 5,000 Example 1 Inventive 99 600 40,000 Example 2

In order to evaluate the fusion resistance of the plated steel sheet thus obtained, the area ratio and thickness of the hard alloy layer, and fusion ratio of the hard alloy layer to the mold, were measured using a simulation device, and the evaluation results are illustrated in Table 2 below.

In this case, the fusion ratio between the plated steel sheet and the mold was determined as follows. After manufacturing a small cup-shaped mold, the surface of the mold before and after hot forming of the plated steel sheet was imaged using a scanning electron microscope, and the area of occurrence of a fused portion was measured using an image analyzer. Therefore, it was evaluated as follows according to the degree of occurrence thereof.

5: Fusion ratio of less than 5%

4: Fusion ratio of 5% or more and less than 10%

3: Fusion ratio of 10% or more and less than 25%

2: Fusion ratio of 25% or more and less than 50%

1: Fusion ratio of 50% or more

Also, in the case of the thickness and the area ratio of the hard alloy layer, the surface and cross section of the plated steel sheet were imaged using a scanning electron microscope, and the area of the alloy layer and the thickness of the alloy layer were measured using an image analyzer, and it was evaluated according to the ratios thereof as follows.

5: Alloy layer area ratio of 30% or more

4: Alloy layer area ratio of 15% or more and less than 30%

3: Alloy layer area ratio of 1% or more and less than 15%

2: Alloy layer area ratio of 0.5% or more and less than 1%

1: Alloy layer area ratio of less than 0.5%

TABLE 2 Al + Zn + Alloy layer Alloy layer Fusion Classification Fe Wt % area ratio thickness (μm) ratio Example of 91 1 0.051 2 related art Comparative 93 1 2.007 2 Example 1 Comparative 59 2 3.368 3 Example 2 Comparative Not Not Not Not Example 3 measureable measurable measurable measurable Inventive 73 4 24.116  4 Example 1 Inventive 99 5 30.103  5 Eample 2

As illustrated in Table 1 and Table 2, in Inventive Examples 1 and 2, in which the plating layer components and hot tempering rolling conditions satisfy the scope of the present disclosure, it can be seen that a hard layer having a predetermined area ratio and thickness is formed on the surface layer portion of the plating layer, and thus fusion resistance is relatively excellent.

For example, as the area ratio of the alloy layer in the inventive examples is 15% or more, which increases by 30 times or more as compared to the related art examples, the fusion ratio is also reduced to less than 10%, and it can be seen that the inventive examples have excellent fusion resistance, compared to the related art examples.

Meanwhile, when plating in the same manner as in the related art example, it can be seen that even when the sum of Al and Zn in the plating layer is 60% or more by weight, a hard alloy phase for improving fusion resistance is not formed on the surface of the plating layer.

In addition, in Comparative Examples 1 and 2, it can be seen that the plating layer component and the hot tempering rolling conditions were outside the scope of the present disclosure, and the alloy layer was not sufficiently formed. In Comparative Example 3, it can be seen that the vaporization of the plating layer and the reaction with the tempering rolling roll proceeded rapidly, so that measurement of the hard phase itself was impossible.

As described above, in the detailed description of the present disclosure, exemplary embodiments of the present disclosure have been described, but various modifications may be made without departing from the scope of the present invention by those of ordinary skill in the art to which the present disclosure pertains. Therefore, the scope of the present disclosure is not limited to the described embodiments and should not be determined thereby, and should be determined by the claims to be described later, as well as those equivalent thereto. 

1. A plated steel sheet for hot forming having excellent fusion resistance, comprising: a plated layer disposed on one surface or both surfaces of a base steel sheet, wherein a surface layer portion of the plated layer is comprised of a hard alloy layer with a surface area ratio of 1% or more including hard alloy phases, the hard alloy layer having a thickness of 0.1-100 μm, and a balance of a soft plating layer, wherein the hard alloy phases comprise Al, Zn, Mg, Si, Fe and a balance of unavoidable impurities, a sum of Al, Zn and Fe being 70 wt % or more on the basis of weight % thereof.
 2. The plated steel sheet for hot forming having excellent fusion resistance of claim 1, wherein the hard alloy layer has an area ratio in a range of 15 to 100%.
 3. The plated steel sheet for hot forming having excellent fusion resistance of claim 1, wherein the soft plating layer includes Al, Zn, Mg, Si and a balance of unavoidable impurities, a sum of Al and Zn being 60% or more on the basis of weight % thereof.
 4. A method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance, comprising: immersing a base steel sheet in a plating bath and plating a surface thereof; and tempering rolling the plated steel sheet discharged from the plating bath and having a plating layer formed on the surface thereof, wherein in the tempering rolling, hot tempering rolling is performed at a pressure of 10 to 50,000 MPa while heating the plated steel sheet at a temperature ranging from 300 to 700° C.
 5. The method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance of claim 4, wherein the plating layer includes Al, Zn, Mg, Si and a balance of unavoidable impurities, a sum of Al and Zn being 60% or more on the basis of weight % thereof.
 6. The method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance of claim 4, wherein a surface layer portion of a plated layer of the temper-rolled plated steel sheet is comprised of a hard alloy layer with an area ratio of 1% or more having a thickness of 0.1 to 100 μm and including hard alloy phases, and a soft plating layer as a remainder thereof, wherein the hard alloy phases include Al, Zn, Mg, Si, Fe, and a balance of unavoidable impurities, a sum of Al, Zn and Fe being 70% or more on the basis of weight % thereof.
 7. The method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance of claim 6, wherein the hard alloy layer has an area ratio in a range of 15 to 100%.
 8. The method of manufacturing a plated steel sheet for hot forming having excellent fusion resistance of claim 4, wherein the plating is performed by immersing the base steel sheet in a hot-dip plating bath and forming a hot-dip plated layer on a surface of the base steel plate. 